1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a light emitting semiconductor device with reducing dislocation density of epitaxial growth.
2. Description of the Related Art
AlInGaN-based compound semiconductor materials are frequently applied to produce light emitting devices such as blue-green light emitting diodes and laser diodes. Theses materials are usually grown on aluminum oxide (Al2O3) or silicon carbide (SiC) substrates.
The semiconductor materials are difficult to directly grow on the substrates because of lattice constant differences. For example, a GaN crystal layer (a=3.189 xc3x85) is hard to directly grow on an aluminum oxide substrate (a=4.758 xc3x85) since the difference of their lattice constants exceeds 16%.
Akasaki et al., in U.S. Pat. No. 4,855,249, first disclosed to grow at a low temperature an amorphous AlN buffer layer on an Al2O3 substrate so as to reduce problems of the lattice constant differences between the Al2O3 substrate and a GaN layer. Nakamura et al., in U.S. Pat. No. 5,290,393, disclosed to use GaN or AlGaN as a buffer layer. An amorphous GaN buffer layer was first grown at a temperature between 400 and 900xc2x0 C. on an Al2O3 substrate. A GaN epitaxy layer was then grown at a temperature between 1000 and 1200xc2x0 C. on the GaN buffer layer. The quality and performance of the GaN epitaxy layer were better than those of a GaN epitaxy layer produced by using AlN as a buffer layer.
Conventionally, nucleuses with one single species are grown between a substrate and a buffer layer so as to balance the lattice constant differences. However, the single-species nucleuses on the substrate still exhibit more occurrence chances of dislocation defects. Please refer to FIGS. 1a to 1d in describing how the dislocation defects tend to occur according to prior art. In FIG. 1a, nucleuses 101 are grown on a substrate 1. Next, a buffer layer 103 is gradually grown on the substrate 1 and the nucleuses 101 as shown in FIG. 1b. After completion of the buffer layer 103, which is shown in FIG. 1c, dislocation defects 104 mostly occur along sides of two nucleuses 101. In FIG. 1d, after growing an epitaxy layer 105, the dislocation defects 104 further extend within the epitaxy layer 105. The dislocation defects 104 reduce both electronic and optical performance of a light emitting device.
There always exists a need to reduce the lattice constant differences between an epitaxy layer and a substrate, ex. between a GaN-based epitaxy layer and an aluminum oxide substrate, since the differences result in dislocation defects of the epitaxy layer and even reduce performance of a semiconductor device thus produced.
The present invention provides a semiconductor device which comprises a monocrystalline substrate, multiple nucleuses on the monocrystalline substrate, a dislocation inhibition layer on the multiple nucleuses, and an epitaxy layer on the dislocation inhibition layer. The multiple nucleuses are made of at least two materials having different crystal constants. The multiple nucleuses are respectively isolated. Preferably, the multiple nucleuses are 10 xc3x85 to 100 xc3x85 thick.